The general procedure for the synthesis of 5-bromo-2-chloro-7H-pyrrolo[2,3-d]pyrimidines from 2-chloro-7H-pyrrolo[2,3-d]pyrimidines is as follows: N-bromosuccinimide (11,374.5 g, 64.04 mol, 2.0 eq.) was added in batches to 2-chloro-7H-pyrrolo[2,3-d]pyrimidines (4907.0 g, 31.91 mol, 1.0 equiv) in acetonitrile solution. The reaction was carried out at room temperature (25 °C) for 17 h. The completion of the reaction was monitored by TLC. Upon completion of the reaction, the reaction solution was slowly poured into 100 L of ice water and a large amount of solid was precipitated. The solid was collected by filtration and the filter cake was washed three times with 50 L of water and dried to give 7117.2 g of white solid product in 95.82% yield. The structure of the product was confirmed by 1H-NMR (400 MHz, DMSO-d6): δ 12.78 (brs, 1H), 8.85 (s, 1H), 7.89 (s, 1H).